1. Field of the Invention
The present invention relates to a driving circuit for a piezoelectric transformer, a cold-cathode tube light-emitting apparatus using a cold-cathode tube as a load of a piezoelectric transformer, a liquid crystal panel in which the cold-cathode tube light-emitting apparatus is built so as to control the brightness, and devices with the liquid crystal panel built therein.
2. Description of the Related Art
Hereinafter, a conventional piezoelectric transformer and driving circuit for the piezoelectric transformer will be described with reference to the drawings.
The piezoelectric transformer has a configuration in which a primary (input) side electrode and a secondary (output) side electrode are formed on a piezoelectric body, an AC voltage in the vicinity of a resonance frequency of the piezoelectric transformer is applied to the primary side electrode to vibrate the piezoelectric transformer mechanically, and the mechanical vibration is converted by a piezoelectric effect so as to be output from the secondary side electrode as a high voltage. The piezoelectric transformer can be rendered smaller and thinner, compared with an electromagnetic transformer, so that a high conversion efficiency can be realized.
FIG. 14 shows a perspective view of a conventional piezoelectric transformer 101 with one output on the secondary side. In FIG. 14, the piezoelectric transformer 101 has the following configuration: primary (input) side electrodes 102 and 103 are formed over substantially half of the principal planes of a rectangular plate 105 made of a piezoelectric ceramic material (e.g., lead zirconate titanate (PZT)) so as to be opposed in the thickness direction, and a secondary (output) side electrode 104 is formed on one end face of the rectangular plate 105 in the length direction. The rectangular plate 105 is previously polarized in the thickness direction using the electrodes 102 and 103, and is previously polarized in the length direction using the electrodes 102, 103 and the electrode 104. When an AC voltage in the vicinity of the resonance frequency of vibration that expands and contracts in the length direction of the piezoelectric transformer 101 is applied between the primary side electrodes 102 and 103, the piezoelectric transformer 101 excites mechanical vibration that expands and contracts in the length direction. The mechanical vibration is converted into a voltage by a piezoelectric effect. The voltage thus obtained is output from the electrode 104 in accordance with an impedance ratio between the primary side electrodes and the secondary side electrode.
FIG. 15 shows a perspective view of a piezoelectric transformer 111 with two outputs on the secondary side. In FIG. 15, the piezoelectric transformer 111 has the following configuration: primary side electrodes 112 and 113 are formed substantially at the center of a rectangular plate 116 (made of a piezoelectric ceramic material) in the length direction so as to be opposed in the thickness direction, a secondary side electrode 114 is formed on one end face of the piezoelectric transformer 111 in the length direction, and another secondary side electrode 115 is formed on the other end face thereof in the length direction. The rectangular plate 116 is previously polarized in the thickness direction using the electrodes 112 and 113, and is previously polarized in the length direction using the electrodes 112, 113 and the electrodes 114 and 115. When an AC voltage in the vicinity of the resonance frequency of vibration that expands and contrasts in the length direction of the piezoelectric transformer 111 is applied between the electrodes 112 and 113, the piezoelectric transformer 111 excites mechanical vibration that expands and contracts in the length direction. The mechanical vibration is converted into a voltage by a piezoelectric effect. The voltage thus obtained is output from the electrodes 114 and 115 in accordance with an impedance ratio between the primary side electrodes and the secondary side electrodes.
Generally, in the piezoelectric transformer, due to the impedance of a load connected to the secondary side, a voltage step-up ratio, which represents a ratio between a voltage input to the primary side and a voltage output from the secondary side, is varied. Furthermore, a driving efficiency represented by the electric power output from the secondary side with respect to the electric power input to the primary side is varied similarly. Therefore, the driving frequency also is varied, which enables the maximum voltage step-up ratio and driving efficiency to be obtained.
For example, in the case of using a cold-cathode tube as a load of the piezoelectric transformer, the cold-cathode tube generally exhibits a high impedance equal to or more than hundreds of MΩ until it lights up, and the impedance decreases rapidly to a range between hundreds of Ω and tens of Ω after it lights up. Furthermore, a voltage equal to or more than several kV is required for allowing the cold-cathode tube to light up, and even during steady lighting, a voltage from hundreds of V to several kV is required. In order to allow the cold-cathode tube to light up efficiently by using the piezoelectric transformer, it is required to change the frequency and the level of an AC voltage applied to the primary side of the piezoelectric transformer between a period before the commencement of lighting and a period after lighting.
As a prior art for realizing the above, a cold-cathode tube driving apparatus described in JP 6(1994)-167694 A is known. FIG. 16 shows a block diagram of a driving apparatus disclosed in this publication.
In FIG. 16, an output signal from a free-running multivibrator 206 is amplified by a current amplifier 207, and a voltage further is stepped-up by a wire-wound transformer 208, if required, to be applied to the primary side of a piezoelectric transformer 201. A cold-cathode tube 202 is connected as a load to a secondary side output of the piezoelectric transformer 201. A current flowing through the cold-cathode tube 202 is detected by a load current detector circuit 209. The detected current is converted to a voltage. The voltage thus obtained is input to one input terminal of an integrator circuit 204 through an AC voltage rectifier circuit 210, and a signal from a variable voltage apparatus 203 is supplied to the other input terminal of the integrator circuit 204. In this manner, an oscillation frequency of the free-running multivibrator 206 is controlled by the integrator circuit 204 through a voltage level shift circuit 205.
A voltage applied to the piezoelectric transformer 201 is set by the variable voltage apparatus 203, the voltage level shift circuit 205, and the like, and the driving frequency of the piezoelectric transformer 201 is swept, whereby the cold-cathode tube 202 that is a load of the piezoelectric transformer 201 is allowed to light up. After lighting, the driving frequency of the piezoelectric transformer 201 again is swept. Furthermore, a voltage applied to the piezoelectric transformer 201 is controlled by the variable voltage apparatus 203, the voltage level shift circuit 205, and the like in accordance with the level of a current detected by the load current detector circuit 209 and the like, whereby the light-emission brightness of the cold-cathode tube 202 is adjusted.
The case where a plurality of loads such as a cold-cathode tube are connected to the piezoelectric transformer is disclosed by, for example, JP 8(1996)-45679 A. In this disclosure, as shown in FIG. 17, cold-cathode tubes 120 and 121 are connected in series to the secondary side electrode 104 of the piezoelectric transformer 101 with one output on the secondary side shown in FIG. 14. In this case, both ends of one cold-cathode tube need to be supplied with a voltage of several kV until the commencement of lighting and a voltage of hundreds of V during steady lighting. Therefore, the piezoelectric transformer 101 is required to output a high voltage based on the number of cold-cathode tubes to be connected.
Therefore, it is required to configure the connection portion and wiring between the piezoelectric transformer 101 and the cold-cathode tube 120, and a circuit board so that they withstand a high voltage. Furthermore, regarding the mounting of circuit components including the piezoelectric transformer on the circuit board, it is required to enlarge the distance between the respective components so as to avoid an insulation breakdown due to a high voltage and enhance safety. Therefore, there is a limit to the enhancement of a mounting density. Even if the piezoelectric transformer and the circuit components are miniaturized, a system including the circuit board cannot be miniaturized and its space cannot be reduced.
Furthermore, there is another conventional example in which a plurality of cold-cathode tubes are connected to the piezoelectric transformer with one output on the secondary side shown in FIG. 14. More specifically, as shown in FIG. 18, the cold-cathode tubes 120 and 121 are connected in parallel to the secondary side electrode 104 of the piezoelectric transformer 101. In this case, due to the variation of impedances of the cold-cathode tubes 120 and 121, a lighting commencement time is varied therebetween. For example, when the cold-cathode tube 120 starts lighting up first, the impedance of the cold-cathode tube 120 rapidly is decreased. Because of this, the voltage step-up ratio of the piezoelectric transformer 101 is decreased, and the cold-cathode tube 121 other than the cold-cathode tube 120 that has started lighting up first cannot keep a voltage level at which lighting can start. As a result, only one cold-cathode tube lights up.
Furthermore, JP 8(1996)-45679 A discloses another conventional example in which a plurality of cold-cathode tubes are connected to the piezoelectric transformer 111 with two outputs on the secondary side shown in FIG. 15. In this disclosure, as shown in FIG. 19, the cold-cathode tubes 120 and 121 are connected in series to the secondary side electrodes 114 and 115 of the piezoelectric transformer 111. Unlike the method shown in FIG. 18, the following state can be avoided: due to the variation of impedances of the cold-cathode tubes 120 and 121, only one cold-cathode tube lights up while the other does not.
Since the connection portion between the cold-cathode tubes 120 and 121 is connected electrically to one primary side electrode 113, non-lighting can be avoided. However, the difference in brightness is caused by the difference in impedance between two cold-cathode tubes during steady lighting. In addition, the piezoelectric transformer 111 needs to perform an unstable operation of continuously supplying different electric powers from two secondary side electrodes 114 and 115.